1. Field of the Invention
The invention relates to a cycloconverter and, in particular, a cycloconverter which feeds a parallel-reasonant circuit as a load.
2. Description of the Prior Art
United States Patent No. 3,599,078 discloses a cycloconverter which comprises an inverter whose output is adapted to be connected to a parallel resonant circuit as the load and whose input is adapted to be connected to an intermediate DC link which is fed by an AC voltage source and which includes a smoothing choke and a controlled rectifier. The inverter of the cycloconverter comprises first and second bridge branches and third and fourth bridge branches, each of which includes a controllable thyristor. An auxiliary commutating device is also provided in the cycloconverter. This commutating device includes first and second converter branches and third and fourth converter branches, each of which branches also includes a controllable thyristor poled in the same direction as the thyristors of the bridge branches. The first and second converter branches are connected in shunt with the first and second bridge branches and a commutation capacitor shunted by a resistor connects the common terminals of the aforesaid bridge and converter branches.
In operation of the above-described known cycloconverter the cycloconverter is load-controlled in the steady state condition and the operating frequency of the inverter is determined by the resonance frequency of the resonant circuit. During such operation, current passes in direct commutation from one bridge branch of the inverter to the bridge branch that carries current next, the reactive commutation power being made available by the capacitor of the parallel resonant circuit. In the starting phase of the cycloconverter, the charge of the load capacitor is not sufficient for direct commutation and indirect commutation via the auxiliary commutation device is necessary during this time.
More particularly, during starting up of the cycloconverter, the thyristors in the first and second converter branches of the auxiliary device and in the first and second bridge branches of the inverter, are fired. These thyristors remain conducting until a given voltage builds up on the commutating capacitor and a given current flows in the choke. During this time, no current flows through the load. Subsequently thyristors of the converter branch that carries current next and of a bridge branch that carries current next and is not connected to the auxiliary commutating device (i.e, the third or fourth bridge branch) are fired, and the previously conducting thyristors are extinguished. At this time, the charge on the commutating capacitor is reversed and a current, which excites an oscillation in the parallel-resonant circuit flows through the load. Thereafter, prior to ech zero crossing of the resonantcurcuit voltage, the thyristors of the bridge branch which conducts next and which is not connected to the commutation capacitor, and the thyristor of the converter branch that conducts next are subsequently fired. As the aforesaid switching cycle is repeated, the voltage at the capacitor of the parallel-resonant circuit becomes larger and larger. When such voltage reaches a value which is sufficient for direct commutation, only thyristors in diagonal bridge branches of the inverter are still fired and the thyristors of the auxiliary commutating device remain unfired. At this point the cycloconverter now operates in normal operation and the residual charge on the commutating capacitor is dissipated in the parallel-connected resistor. As is apparent from the above, the aforesaid cycloconverter is not load-controlled during the start-up phase, but is self-commutated at the frequency of the resonant-circuit voltage. This result, however, is achieved by means of a very complicated control comprising complicated logic circuitry.
It is an object of the present invention to provide a cycloconverter of the above type which is designed so as to permit improved and simpler operation during its start-up phase.